Uncertainties in the Evolution of South Indian Ocean Subsurface Heat Content in CMIP6 models: The Role of the Indonesian Throughflow and Its Pacific Drivers

The Subtropical Indian Ocean (STIO) has emerged as a prominent warming hotspot since the 1950s under anthropogenic forcing. However, coupled climate models have struggled to reproduce the observed spatial distribution and magnitude of this warming. An analysis using 30 CMIP6 models reveals that the largest inter-model uncertainty in long-term ocean heat uptake over the STIO. In this study, we investigate the causes of this spread, primarily focusing on the role of the Indonesian Throughflow (ITF) and its Pacific linkages. CMIP6 models exhibit large inconsistencies in simulating ITF induced upper ocean (top 700 m) heat content variability in the STIO. A regression-based ITF model (ITF-PAC) using canonical ENSO and ENSO-Modoki indices broadly reproduces a similar relationship between the Pacific climate modes and the STIO heat content, but with a much stronger correlations in models than in reanalysis, suggesting an exaggerated Pacific control in the CMIP6 models. Our results highlight these coupled models possess low skill and large uncertainty in capturing ITF-driven heat content variability and long-term Pacific trends that can modulate STIO warming.